How this works
Aspect ratio is the proportional relationship between width and height. It's a shape descriptor, not a size descriptor: 1920×1080 and 1280×720 both share 16:9, just at different sizes. Designers, photographers, and video editors use aspect ratio constantly — every export, crop, and resize either preserves or deliberately changes it.
The common ratios you'll meet: **16:9** for HD/4K video and most modern monitors; **4:3** for older monitors and traditional broadcast TV; **21:9** for ultrawide cinema and gaming displays; **3:2** for DSLR sensors (and 35mm film); **1:1** for Instagram squares and many product images; **9:16** for mobile-first vertical video (TikTok, Reels). Print uses 4:5 portrait (8×10") and 5:7 frequently, neither of which matches typical sensor or screen ratios — which is why printing usually involves cropping.
Resizing without losing aspect ratio is the most common task. Pick the new width *or* height; multiply by the current ratio to find the other. Stretching to a new ratio without cropping causes distortion (squashed or stretched faces). The right move is either to crop to the target ratio first, or to letterbox/pillarbox with bars, or to crop intelligently around the subject. This calculator handles the math — your creative judgement handles the cropping.
The formula
W = current width, H = current height, gcd = greatest common divisor (Euclidean algorithm). Ratios with non-integer simplification (e.g. 1366 × 768 simplifies to 683:384, not the marketed 16:9) reveal "almost-but-not-quite" panel dimensions.
Example calculation
- Image 1920 × 1080.
- gcd = 120 → ratio 16:9. Resize to width 800: height = 800 × (1080/1920) = 450 px.
Frequently asked questions
Why does 1366×768 not simplify to 16:9?
1366 / 768 = 1.7786, which is close to 16/9 = 1.7778 but not exact. The actual simplified ratio is 683:384. Cheap laptop panels have used 1366 × 768 for years because it's a manufacturing convenience that's "close enough" to be marketed as 16:9. True 16:9 at HD-class is 1280 × 720 or 1920 × 1080. The visual difference is invisible to the eye, so it doesn't matter unless you're cropping pixel-perfect graphics for that exact panel.
How do I convert from 4:3 to 16:9 without distortion?
Three options: (1) **Crop** — chop off the top and bottom of the 4:3 frame to make it 16:9. Loses content but no bars. (2) **Pillarbox** — keep the 4:3 inside a 16:9 canvas with black bars on the sides. Preserves everything but adds dead space. (3) **Stretch** — scale 4:3 horizontally to 16:9 width. Distorts faces (avoid). For most modern uses, crop the centre or whatever has the subject. Software like Premiere offers "auto reframe" that uses motion tracking to keep the subject in frame.
What's the right ratio for social media?
A current cheat sheet: **Instagram feed** 1:1 (square) or 4:5 (portrait, takes up more screen). **Instagram Reels / TikTok / YouTube Shorts** 9:16 (vertical). **YouTube standard** 16:9 (horizontal). **Twitter/X feed** 16:9 plays well; 1:1 is also fine. **LinkedIn** 1.91:1 (close to 1.91 wide). **Facebook feed** 16:9 or 1:1. When in doubt, post 1:1 — it adapts to most placements without cropping. Vertical 9:16 is now the dominant short-form video shape.